Controller of switching power supply and control method thereof

ABSTRACT

A switching power supply controller and a control method thereof are disclosed in the invention. By adjusting a switching frequency of the switching power supply based on a radio of a switching period of the power transistor to the sum of the conduction time of the power transistor and the demagnetization time of an inductor, the switching frequency of the switching power supply varies within a preset range under different load conditions. The switching power supply controller of the present invention prevents the effect of the frequency jittering control from being weakened due to the decrease of the switching frequency under heavy load, thereby improving the conducted electromagnetic interference.

FIELD OF THE INVENTION

The present invention relates to the field of power electronics, and more particularly, to a controller of a switching power supply and a control method thereof.

BACKGROUND

A switching power supply usually produces serious electromagnetic interference (EMI) due to fast switching of power transistors in operation. It is a common concern in industry to reduce EMI as required by some standards of the power supply. A conventional approach to reduce EMI of the switching power supply is to add an EMI filter into the circuit, which, however, increases cost and size of the power supply, deteriorates high-frequency performance, and increases power consumption.

The frequency jittering control is an effective method for suppressing conducted EMI, and has been widely used in some low-power switching power supplies. The method changes an operating frequency of the switching power supply periodically or non-periodically in a range, so that harmonic interference energy is dispersed and the EMI standards are met.

However, the conventional frequency jittering control in the switching power supply is based on the principle of changing a system switching frequency of the switching power supply. Although it can suppress conducted EMI, the power transistor will not be turned on at idea moment due to variation of the system switching frequency. The frequency jittering control leads to higher conduction loss, and lower system efficiency and higher heat dissipation. Moreover, because the power transistor is turned on and off at a high voltage, additional electromagnetic radiation may occur, which results in degradation of EMI.

Thus, it is desirable to further improve the frequency jittering control in a switching power supply to reduce conduction loss and suppress electromagnetic radiation of the power transistor

SUMMARY

In view of the above, the purpose of the present invention is to provide a switching power supply controller and a control method thereof, in order to improve EMI by adopting the frequency jittering control.

In the first aspect, a controller of a switching power supply is provided. The controller comprises:

a frequency-jittering control circuit, configured to generate a frequency-jittering signal adaptively;

a superimposing circuit, configured to superimpose the frequency-jittering signal on a feedback control signal or an inductor current sampling signal to generate a superimposed signal; and

a comparator, configured to compare the superimposed signal against the inductor current sampling signal or the feedback control signal to generate a switching control signal, in order to control a power transistor in the switching power supply, wherein a switching frequency of the switching power supply varies within a preset range under different load conditions.

In an embodiment, an amplitude of the frequency-jittering signal is proportional to a switching period of the power transistor, and inversely proportional to the sum of a conduction time of the power transistor and a demagnetization time of an inductor, such that under different load conditions the switching frequency of the switching power supply varies within the preset range.

In an embodiment, an amplitude of the frequency-jittering signal is proportional to a radio of a switching period of the power transistor to the sum of a conduction time of the power transistor and a demagnetization time of an inductor, such that under different load conditions the switching frequency of the switching power supply varies within the preset range.

In an embodiment, an amplitude of the frequency-jittering signal is proportional to the feedback control signal.

In an embodiment, the frequency-jittering signal is one of a triangular wave and a sinusoidal wave.

In an embodiment, when a load is between a first threshold value and a full load, the frequency-jittering signal is generated adaptively, such that the switching frequency of the switching power supply varies within the preset range.

In an embodiment, the frequency-jittering control circuit comprise:

an error amplifier, having a non-inverting input terminal for receiving the feedback control signal and an inverting input terminal for receiving an inverting input signal proportional to an intermediate signal; and being configured to generate the intermediate signal representative of a amplitude of the frequency-jittering signal.

In an embodiment, the frequency-jittering control circuit further comprises:

an inverting input signal generating circuit, configured to multiply the intermediate signal with a proportional coefficient to generate the inverting input signal, wherein the proportional coefficient is a radio of a switching period of the power transistor, to the sum of a conduction time of the power transistor and a demagnetization time of an inductor.

In an embodiment, the frequency-jittering control circuit further comprises

a frequency-jittering signal generating circuit, configured to generate the frequency-jittering signal based on the intermediate signal, wherein a peak-to-peak value of the frequency-jittering signal is an amplitude of the intermediate signal, and the frequency-jittering signal is a triangular wave.

In an embodiment, the inverting input signal generating circuit comprises a switch circuit and a filter circuit,

wherein the switch circuit is configured to transmit the intermediate signal to the filter circuit during a first period of a switching period of the power transistor; and the filter circuit is grounded during a second period of the switching period, the first period is the sum of the conduction time of the power transistor and the demagnetization time of an inductor, and the second period is other time periods except the first time period in the switching period.

In an embodiment, the switch circuit comprises:

a first switch having a first terminal for receiving the intermediate signal, and a second terminal coupled to an input terminal of the filter circuit; and

a second switch having a first terminal coupled to the input terminal of the filter circuit, and a second terminal coupled to a ground reference; wherein the first switch and the second switch are complementarily turned on, and the first switch is turned on during the first period, and the second switch is turned on during the second period.

In a second aspect, a control method of a switching power supply is provided. The control method comprises: generating a frequency-jittering signal adaptively;

superimposing the frequency-jittering signal on a feedback control signal or an inductor current sampling signal to generate a superimposed signal; and

generating a switching control signal by comparing the superimposed signal against the inductor current sampling signal or the feedback control signal, in order to control a power transistor in the switching power supply, wherein a switching frequency of the switching power supply varies within a preset range under different load conditions.

In an embodiment, an amplitude of the frequency-jittering signal is proportional to a switching period of the power transistor, and inversely proportional to the sum of a conduction time of the power transistor and a demagnetization time of an inductor, such that under different load conditions the switching frequency of the switching power supply varies within the preset range.

In an embodiment, an amplitude of the frequency-jittering signal is proportional to the feedback control signal.

A switching power supply controller and a control method thereof are disclosed in the invention. By adjusting a switching frequency of the switching power supply based on a radio of a switching period of the power transistor to the sum of the conduction time of the power transistor and the demagnetization time of an inductor, the switching frequency of the switching power supply varies within a preset range under different load conditions. The switching power supply controller of the present invention prevents the effect of the frequency jittering control from being weakened due to the decrease of the switching frequency under heavy load, thereby improving the conducted electromagnetic interference.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present disclosure are apparent from the following description of the embodiments of the present disclosure with reference to the drawings. In the drawings:

FIG. 1 is a schematic diagram of an example switching power supply;

FIG. 2 is a schematic diagram of a first example controller of a switching power supply;

FIG. 3 is a schematic diagram of a second example controller with frequency-jittering control of a switching power supply;

FIG. 4 is a schematic diagram of a first example controller for a switching power supply, in accordance with the embodiments of the present invention;

FIG. 5 is a waveform diagram of example operation of the switching power supply, in accordance with the embodiments of the present invention;

FIG. 6 is a schematic diagram of a second example controller of a switching power supply, in accordance with the embodiments of the present invention.

DETAILED DESCRIPTION

The present disclosure is described below based on embodiments, and the present disclosure is not limited to the embodiments. In the following description of the present disclosure, specific details are described in detail. The present disclosure can be fully understood by those skilled in the art without the description of these details. Well-known methods, processes, flows, components and circuits are not described in detail to avoid obscuring the essence of the present disclosure.

Further, it should be understood by those skilled in the art that, the drawings herein are only for illustration, and are not necessarily drawn to scale.

It should further be understood that in the following description, a “circuit” indicates a conductive loop formed by at least one component or sub-circuit by electrical connection or electromagnetic connection. In a case that a component or circuit is referred to as being “connected to” another component or being “connected” between two nodes, the component or circuit may be coupled or connected to another component directly or via an intermediate component. The connection between the two components may be physical connection, logical connection, or a combination thereof. In addition, in a case that the component is referred to as being “directly coupled to” or “directly connected to” another component, it is indicated that no intermediate component exists between the two components.

Unless otherwise indicated in the context, the words “including”, “comprising”, and the like, in the specification and the claims, should be interpreted as inclusive rather than being exclusive or exhaustive, i.e., “including but not limited to”.

In the description of the present disclosure, it should be understood that, the terms “first”, “second”, and the like are only for descriptive purposes and should not be construed as indicating or implying relative importance. In addition, in the description of the present disclosure, the term “multiple” indicates two or more unless otherwise specified.

Referring now to FIG. 1, shown is a schematic diagram of an example switching power supply. In this example, the switching converter is a flyback switching power supply, including transformer T. Transformer T includes primary winding Np, secondary winding Ns and auxiliary winding Na configured to transfer power energy and feedback a signal. At the primary side, an external AC input signal after rectified and filtered can be configured as input voltage Vin. A first terminal of primary winding Np of transformer T can receive input voltage Vin, and power transistor S and sampling resistor Rs are connected in series between a second terminal of primary winding Np of transformer T and the ground. At the secondary side, a first terminal of secondary winding Ns of transformer T can connect to diode D, and capacitor Co can connect in parallel between a cathode of diode D and the ground, and output voltage Vo can be generated at both terminals of capacitor Co.

A first terminal of auxiliary winding Na of transformer T can be configured as a feedback terminal, and a second terminal of auxiliary winding Na can be grounded. Resistors R1 and R2 can form a voltage divider network, and voltage feedback signal V_(FB) representative of output voltage Vo at the secondary side can be obtained from the first terminal of auxiliary winding Na. A voltage feedback circuit can obtain voltage feedback signal V_(FB) from auxiliary winding Na of transformer T. Controller 10 can obtain inductor current sampling signal Vcs representative of a current flowing through primary winding Np from sampling resistor Rs, and obtain voltage feedback signal V_(FB) of the secondary voltage at secondary winding Ns from the voltage feedback circuit. Controller 10 can generate switching control signal Vg according to inductor current sampling signal Vcs and voltage feedback signal V_(FB), thereby controlling the switching state of power transistor S. For example, switching control signal Vg can be configured as a pulse width modulation (PWM) signal with a duty cycle, such that power transistor S can be switched between an on state and an off state. Here, it should be understood that the embodiment of the present invention only provides a solution for obtaining inductor current sampling signal Vcs and voltage feedback signal V_(FB). In other embodiments, other possible methods may be adopted to obtain the sampling or feedback signal in the circuit. When power transistor S is turned on, primary winding Np of transformer T can bear input voltage Vin, and primary inductor current IL rises linearly from zero. When inductor current sampling signal Vcs generated across sampling resistor Rs reaches feedback control signal Vcomp, power transistor S is turned off. Here feedback control signal Vcomp represents a limit value of the primary inductor current at the primary side. In one embodiment, feedback control signal Vcomp can characterize an error between voltage feedback signal V_(FB) representative of the output voltage at the secondary side and a reference voltage. When power transistor S is turned off, a freewheeling current flows through secondary diode D, the secondary winding has a negative voltage Vo. and primary inductor current IL decreases linearly. In the quasi-resonant mode, power transistor Scan be turned on again at a valley of its source-drain voltage. In the quasi-resonant mode, feedback control signal Vcomp can determine a conduction time of power transistor S and the freewheeling current duration of secondary diode D, that is, feedback control signal Vcomp can determine the switching frequency of power transistor S.

Referring now to FIG. 2, shown is a schematic diagram of a first example controller of a switching power supply. This controller 10 can include comparator 101 and RS flip-flop 102. A non-inverting input terminal of comparator 101 can receive inductor current sampling signal Vcs, and an inverting input terminal of comparator 101 can receive feedback control signal Vcomp. An output terminal of comparator 101 can provide a turn-off signal. Set terminal S of RS flip-flop 102 can receive a turn-on signal, reset terminal R can receive the turn-off signal, and output terminal Q of RS flip-flop 102 can generate switching control signal Vg. During the conduction period of power transistor S, inductor current sampling signal Vcs can linearly rises. When inductor current sampling signal Vcs reaches feedback control signal Vcomp, the turn-off signal has a high level, and the signal at output terminal Q of RS flip-flop 102 has a low level. After power transistor S is turned off, when the turn-on signal has a high level again, the RS flip-flop can provide switching control signal Vg with a high-level, such that power transistor S can be turned on again. Then, the next switching period starts.

Referring now to FIG. 3, shown is a schematic diagram of a second example controller with frequency-jittering control of a switching power supply. In the conventional frequency-jittering control, by adding frequency-jittering signal Vjitter to feedback control signal Vcomp to change the amplitude of feedback control signal Vcomp, the switching frequency can be changed, thereby realizing the frequency jittering control. As we all know, the switching power supply can produce electromagnetic interference when operating at a fixed frequency, and the conducted interference is mainly concentrated in the frequency points of 1, 2, and 3 times switching frequency. When the switching frequency is very fixed, the energy at the 1, 2, and 3 times frequency points is very concentrated. In the conduction test, the spikes at the above-mentioned frequency points are very high, and a stronger filter circuit must be adopted to meet the electromagnetic compatibility requirements.

In one frequency-jittering control method, the amplitude of frequency jittering signal Vjitter is fixed at 30%-100% load. In this way, this method has two disadvantages: one is at 30%-50% load, the frequency-jittering control will cause larger ripples; second, when at 25% and 10% loads no frequency jittering control is involved, which leads to conduction interference that cannot be optimized. In another existing frequency-jittering control method, the amplitude of frequency jittering signal Vjitter can be determined according to the load condition. That is, the amplitude of the frequency-jittering signal is large under heavy load, and the amplitude of the frequency jittering signal is small under light load. This method also has two disadvantages: one is that a small amplitude of the frequency jittering signal at light load will cause poor frequency jittering effect; second, at 25% and 10% loads, no frequency jittering is involved, which leads to conduction interference that cannot be optimized. In order to solve the above-mentioned problems, the present invention proposes a new frequency jittering control method. When the load is between a first threshold to a full load (e.g., 30%-100% heavy load), the controller can generate the frequency jittering signal adaptively, such that under different load conditions, the switching frequency can vary with a preset range, that is, the jittering range of the switching frequency is fixed.

Referring now to FIG. 4, shown is a schematic diagram of a first example controller for a switching power supply, in accordance with the embodiments of the present invention. This controller 30 can include comparator 301, RS flip-flop 302, frequency jittering control circuit 303, and superimposing circuit 304. Frequency jittering control circuit 303 can generate frequency jittering signal Vjitter2 adaptively, such that the jittering range of switching frequency f of the power transistor in the switching power supply can be fixed under different load conditions, that is, the switching frequency can vary with the preset range. Superimposing circuit 304 can superimpose feedback control signal Vcomp on frequency jittering signal Vjitter2 to generate superimposed signal Vcomp1. Comparator 301 can compare superimposed signal Vcomp1 with inductor current sampling signal Vcs, and generate the turn-off signal for controlling the power transistor in the switching power supply to be turned off when inductor current sampling signal Vcs rises to superimposed signal Vcomp1. A non-inverting input terminal of comparator 301 can receive inductor current sampling signal Vcs, an inverting input terminal of comparator 301 can receive superimposed signal Vcomp1, and an output terminal of comparator 301 can generate turn-off signal Vr. Set terminal S of RS flip-flop 302 can receive turn-on signal Vs, reset terminal R of RS flip-flop 302 can receive turn-off signal Vr, and output terminal Q of RS flip-flop 302 can generate switching control signal Vg. Frequency jittering control circuit 303 can control the amplitude of frequency jittering signal Vjitter2 to be proportional to switching period T of power transistor S, and to be inverse proportional to the sum of conduction time Ton of power transistor S and demagnetization time Tdis of the inductor, such that the jittering range of the switching frequency can be fixed under different load conditions. That is, the amplitude of frequency jitter signal Vjitter2 is proportional to the ratio of switching period T to the sum of conduction time Ton and demagnetization time Tdis of the inductor, such that under different load conditions, the jittering range of the switching frequency can be unchanged. Further, the amplitude of frequency-jittering signal Vjitter2 is also proportional to feedback control signal Vcomp. In this embodiment, frequency jittering signal Vjitter2 can be configured as a triangular wave signal, and the amplitude of frequency jittering signal Vjitter2 refers to a peak-to-peak value of the triangular wave signal. In other embodiments, the frequency-jittering signal Vjitter2 may also be a sine wave signal, as long as the amplitude of its peak-to-peak value meets the foregoing relationship.

FIG. 5 is a waveform diagram of example operation of the switching power supply, in accordance with the embodiments of the present invention. The waveform of drain-source voltage Vds of power transistor S of the flyback power supply is shown here. During conduction time Ton, power transistor S is turned on, and drain-source voltage Vds is close to zero, and during demagnetization time Tdis, the voltage of secondary winding Ns of transformer T is clamped to output voltage Vo, and the energy stored in transformer T is released to the output terminal of the flyback power supply through diode D. After demagnetization time Tdis is over, diode D does not conduct in the reverse direction, thus secondary winding Ns is not clamped, and transformer T resonates with the parasitic capacitor, and the next switching period starts at an appropriate moment. In addition, at the end of demagnetization time Tdis, drain-source voltage Vds being less than a certain threshold can be detected easily, such that demagnetization time Tdis can be obtained.

Frequency jittering control circuit 303 can include error amplifier 3031, inverted input signal generating circuit 3032, and frequency jittering signal generating circuit 3033. Error amplifier 3031 can generate intermediate signal Vjitter1 representative of the amplitude of frequency jitter signal Vjitter2. A non-inverting input terminal of error amplifier 3031 can receive feedback control signal Vcomp, and an inverting input terminal of error amplifier 3031 can receive an inverting input signal proportional to intermediate signal Vjitter1. Here, it should be understood that the non-inverting input terminal of error amplifier 3031 can receive the product of feedback control signal Vcomp and proportional coefficient k obtained after voltage division. For example, proportional coefficient k is R4/(R3+R4), and the signal received by the non-inverting input terminal of error amplifier 3031 is Vcomp*k. Inverting input signal generating circuit 3032 can multiply intermediate signal Vjitter1 with the adaptive proportional coefficient to obtain inverting input signal Vinv. An inverting input terminal of error amplifier 3031 can receive inverting input signal Vinv. Here, proportional coefficient k is the ratio of the sum of conduction time Ton of power transistor S and demagnetization time Tdis of the inductor to switching period T. Inverting input signal generating circuit 3032 can include a switch circuit and a filter circuit, as shown in FIG. 4. The switch circuit can transmit intermediate signal Vjitter1 to the filter circuit during first time period T1 of power transistor S. The filter circuit cannot be enabled during second time period. Here, second time period T2 is the time period other than first time period T1 in switching period T. First time period T1 can be the sum of conduction time Ton of power transistor S and demagnetization time Tdis of the inductor, and the sum of first time period T1 and second time period 12 is switching period T of power transistor S. Inverting input signal Vinv can be generated at the output terminal of the filter circuit. The switch circuit can include first switch K1 and second switch K2. A first terminal of first switch K1 can receive intermediate signal Vjitter1, a second terminal of first switch K1 is connected to the input terminal of the filter circuit, and the first terminal of second switch K2 is connected to the input terminal of the filter circuit, the second terminal of second switch K2 is connected to the ground. In this example, first switch K1 and second switch K2 are complementarily turned on, and first switch K1 is turned on during first time period T1, and second switch K2 is turned on during second time period T2. The filter circuit can be an RC filter composed of resistor R5 and capacitor C1, wherein the first terminal of resistor R5 is the input terminal of the filter circuit and is connected to first switch K1, the second terminal of resistor R5 is connected to capacitor C1, and the other terminal of capacitor C1 is connected to the reference ground. Signal Vinv is generated at the common node of resistor R5 and capacitor C1. It is should be understood that the filter voltage can also be implemented by filters of other structures. The frequency jittering signal generating circuit 3033 can generate a triangular wave signal with a peak-to-peak value of intermediate signal Vjitter1 as frequency jitter signal Vjitter2 according to intermediate signal Vjitter1. In one embodiment, the value of frequency jittering signal Vjitter2 varies between −½ Vjitter1˜½ Vjitter1; in other embodiments, the value of frequency jittering signal Vjitter2 can also change between 0˜+Vjitter1 or 0˜−Vjitter1. In other embodiments, the frequency jitter signal Vjitter2 may also be a sine wave signal, and is not necessarily limited to the triangular wave signal in the embodiment of the present invention.

This embodiment provides a frequency jittering method for adjusting the switching frequency by changing the conduction time of the power transistor. Since frequency jittering signal Vjitter2 is superimposed on feedback control signal Vcomp to control conduction time Ton of the power transistor, according to the volt-second balance rule, demagnetization time Tdis can change with the change of conduction time Ton, and switching period T can change accordingly, such that the switching frequency can be changed. The following further analyzes the working principle of frequency jittering control circuit 303 according to FIG. 4. The jittering range of the switching frequency (period) is shown in formula 1, AT is the jitter range of switching period T, ΔVcomp is the jittering amplitude of feedback control signal Vcomp.

ΔT/T=ΔVcomp/Vcomp*(Ton+Tdis)/T  (1)

In FIG. 4, the jittering amplitude of feedback control signal Vcomp is:

Vjitter1*(Ton+Tdis)T=Vcomp*k  (2).

Formula 2 can be converted to the following formula 3;

Vjitter1=Vcomp*k*T/(Ton+Tdis)  (3).

Since intermediate signal Vjitter1 in formula 3 is ΔVcomp in formula 1, ΔVcomp can be replaced by intermediate signal Vjitter1, and formula 4 can be obtained;

ΔT/T=k  (4)

Therefore, it can be known that no matter the load is 30%, 50% or 100%, the jittering amplitude of the frequency jittering signal can be adjusted adaptively based on the ratio T/(Ton+Tdis), that is, the ratio of switching period T to the sum of conduction time Ton and demagnetization time Tdis, such that the jittering amplitude of the frequency jitter signal can be adaptively determined according to the load, and the jittering range of the switching period is constant value k. The controller of the present invention prevents the effect of the frequency jitter control from being weakened due to the decrease of the switching frequency under heavy load, thereby improving the conducted electromagnetic interference.

Referring now to FIG. 6, shown is a schematic diagram of a second example controller of a switching power supply, in accordance with the embodiments of the present invention. This switching power supply controller 40 can include comparator 401, RS flip-flop 402, frequency jittering control circuit 403, and superimposing circuit 404. The structure and operating principle of RS flip-flop 402 and frequency jittering control circuit 403 are respectively the same as those of RS flip-flop 302 and frequency jittering control circuit 303 in the above embodiment, and will not be repeated here. The difference between this embodiment and the above embodiment is that superimposing circuit 404 can be configured to superimpose inductor current sampling signal Vcs and frequency jittering signal Vjitter2 to generate superimposed signal Vcs1. Further, comparator 401 can compare superimposed signal Vcs1 against feedback control signal Vcomp, and when superimposed signal Vcs1 rises to feedback control signal Vcomp, the switching control signal can be generated to control power transistor Sto be turned off.

This controller 40 can provide a frequency jittering method for changing the switching frequency by adjusting the conduction time of the power transistor in the switching power supply. In this example, conduction time Ton of the power transistor can be adjusted by superimposing frequency jittering signal Vjitter2 on inductor current sampling signal Vcs. Based on the volt-second balance rule, demagnetization time Tdis can change with the change of conduction time Ton, such that switching period T can be adjusted accordingly. Further, the jitter amplitude of the frequency jittering signal can be adaptively adjusted according to the ratio T/(Ton+Tdis), that is, the ratio of switching period T of power transistor S to the sum of conduction time Ton and demagnetization time Tdis of the inductor, such that the jitter amplitude of the frequency jitter signal can be adaptively determined according to the load, and the jittering range of the switching period is constant value k. The controller of the present invention prevents the effect of the frequency jitter control from being weakened due to the decrease of the switching frequency under heavy load, thereby improving the conducted electromagnetic interference. Those skilled in the art should understand that there are different circuit variations within the scope of the present invention. The circuit components shown in the embodiments may be placed in different arrangements or orders, but still fall within the scope of the present invention and provide the functions described by the circuits originally arranged or ordered in the described embodiments.

The foregoing are merely preferred embodiments of the present disclosure and are not intended to limit the present disclosure, and those skilled in the art can make various modifications and variations to the present disclosure. Any modifications, equivalent substitutions and improvements within the spirit and the principle of the present disclosure fall in the protection scope of the present disclosure. 

What is claimed is:
 1. A controller of a switching power supply comprising: a frequency-jittering control circuit, configured to generate a frequency-jittering signal adaptively; a superimposing circuit, configured to superimpose the frequency-jittering signal on a feedback control signal or an inductor current sampling signal to generate a superimposed signal; and a comparator, configured to compare the superimposed signal against the inductor current sampling signal or the feedback control signal to generate a switching control signal, in order to control a power transistor in the switching power supply, wherein a switching frequency of the switching power supply varies within a preset range under different load conditions.
 2. The controller according to claim 1, wherein an amplitude of the frequency-jittering signal is proportional to a switching period of the power transistor, and inversely proportional to the sum of a conduction time of the power transistor and a demagnetization time of an inductor, such that under different load conditions the switching frequency of the switching power supply varies within the preset range.
 3. The controller according to claim 1, wherein an amplitude of the frequency-jittering signal is proportional to a radio of a switching period of the power transistor to the sum of a conduction time of the power transistor and a demagnetization time of an inductor, such that under different load conditions the switching frequency of the switching power supply varies within the preset range.
 4. The controller according to claim 2 or 3, wherein an amplitude of the frequency-jittering signal is proportional to the feedback control signal.
 5. The controller according to claim 2 or 3, wherein the frequency-jittering signal is one of a triangular wave and a sinusoidal wave.
 6. The controller according to claim 1, wherein when a load is between a first threshold value and a full load, the frequency-jittering signal is generated adaptively, such that the switching frequency of the switching power supply varies within the preset range.
 7. The controller according to claim 1, wherein the frequency-jittering control circuit comprise: an error amplifier, having a non-inverting input terminal for receiving the feedback control signal and an inverting input terminal for receiving an inverting input signal proportional to an intermediate signal; and being configured to generate the intermediate signal representative of a amplitude of the frequency-jittering signal.
 8. The controller according to claim 7, wherein the frequency-jittering control circuit further comprises: an inverting input signal generating circuit, configured to multiply the intermediate signal with a proportional coefficient to generate the inverting input signal, wherein the proportional coefficient is a radio of a switching period of the power transistor, to the sum of a conduction time of the power transistor and a demagnetization time of an inductor.
 9. The controller according to claim 7, wherein the frequency-jittering control circuit further comprises a frequency-jittering signal generating circuit, configured to generate the frequency-jittering signal based on the intermediate signal, wherein a peak-to-peak value of the frequency-jittering signal is an amplitude of the intermediate signal, and the frequency-jittering signal is a triangular wave.
 10. The controller according to claim 8, wherein the inverting input signal generating circuit comprises a switch circuit and a filter circuit, wherein the switch circuit is configured to transmit the intermediate signal to the filter circuit during a first period of a switching period of the power transistor; and the filter circuit is grounded during a second period of the switching period, the first period is the sum of the conduction time of the power transistor and the demagnetization time of an inductor, and the second period is other time periods except the first time period in the switching period.
 11. The controller according to claim 10, wherein the switch circuit comprises: a first switch having a first terminal for receiving the intermediate signal, and a second terminal coupled to an input terminal of the filter circuit; and a second switch having a first terminal coupled to the input terminal of the filter circuit, and a second terminal coupled to a ground reference; wherein the first switch and the second switch are complementarily turned on, and the first switch is turned on during the first period, and the second switch is turned on during the second period.
 12. A control method for a switching power supply, comprising: generating a frequency-jittering signal adaptively; superimposing the frequency-jittering signal on a feedback control signal or an inductor current sampling signal to generate a superimposed signal; and generating a switching control signal by comparing the superimposed signal against the inductor current sampling signal or the feedback control signal, in order to control a power transistor in the switching power supply, wherein a switching frequency of the switching power supply varies within a preset range under different load conditions.
 13. The control method according to claim 12, wherein an amplitude of the frequency-jittering signal is proportional to a switching period of the power transistor, and inversely proportional to the sum of a conduction time of the power transistor and a demagnetization time of an inductor, such that under different load conditions the switching frequency of the switching power supply varies within the preset range.
 14. The control method according to claim 12, wherein an amplitude of the frequency-jittering signal is proportional to the feedback control signal. 